1. Field of the Invention
The invention is based on a process for producing a workpiece from a chromium alloy in accordance with the preamble of the first claim. The invention also relates to the use of the workpiece produced using the process.
2. Discussion of Background
In power engineering, in particular for retaining rings in the construction of turbine generators, in offshore engineering, in aeronautical and aerospatial engineering, in architecture, in general mechanical engineering, in the chemical industry and in transport engineering, there is a need for materials which, in addition to having a very high strength, toughness and freedom from ferromagnetism, are also free from susceptibility to corrosion and stress corrosion cracking, both in water and in aqueous halide solutions. However, materials which fulfill all these conditions to a satisfactory extent have not yet been discovered. Therefore, in each instance it is attempted to select materials for the particular application field in such a way that at least the most important properties are covered, in order to prevent failure of the material. In so doing, it is accepted that when operating conditions change subsidiary properties of the material, which have not been taken into account to a sufficient extent, may cause the material to fail.
For retaining rings used in the construction of turbine generators, steels of the composition 18% Cr, 18% Mn, 0.6% N or 18% Mn, 5% Cr, 0.55% C, for example, are employed. Although these materials do have the desired high strength, toughness and freedom from ferromagnetism, their corrosion and stress corrosion cracking properties may become a problem under particularly corrosive operating and environmental conditions.
EP 0,657,556 A1 has disclosed an alloy of the composition:
remainder iron and production-related admixtures and impurities.
Although these alloys which are described in EP 0,657,556 A1 do have a desired high level of resistance to general corrosion, their yield strengths (xe2x80x9celongation limitsxe2x80x9d) only reach a maximum of about 500 MPa and their tensile strengths only reach approximately 850 MPa. However, this is not sufficient for the abovementioned demands for extremely high strengths which are fulfilled by the alloys described previously.
The alloy described in EP 0,657,556 A1 is marketed by the company Krupp VDM under the name Nicrofer(copyright) 3033xe2x80x94alloy 33. The associated materials data sheet, Krupp VDM, Nicrofer(copyright) 3033xe2x80x94alloy 33, materials data sheet No. 4142, June 1995 issue, states that workpieces, following 15% cold deformation, should be subjected to a heat treatment which is to be carried out at temperatures of from 1080 to 1150xc2x0 C., preferably at 1120xc2x0 C. In order to achieve optimum corrosion properties, following the heat treatment cooling is to be accelerated by means of water. Following the heat treatment, the workpieces have the low strength properties described above.
Accordingly, one object of the invention, using a process for producing a workpiece from a chromium alloy of the type mentioned at the outset, is to provide a material of high strength and toughness and with a high level of freedom from ferromagnetism and freedom from susceptibility to stress corrosion cracking, both in water and in aqueous halide solutions.
The essence of the invention is therefore that the workpiece is cold worked and, by means of the cold working, is brought to a yield strength of at least 1000 MPa (Rpxe2x89xa71000 MPa).
The advantages of the invention are to be seen, inter alia, in the fact that degrees of cold deformation (reduction in cross section as a result of cold working) of 20 percent and more, up to over 90 percent, bring about excellent combinations of mechanical, physical and chemical properties. It is thus possible to achieve yield strengths of from 1000 MPa to well over 2000 MPa while still retaining a good level of toughness (elongation at break of from 5 to over 10 percent). The result is a material of extremely high strength which is able to satisfy the demands of modern engineering.
A further advantage is the particular physical and chemical properties, which are not to be found in conventional materials of the same strength and the same resistance to corrosion. The particular physical properties of the material according to the invention emerge essentially in the absence of ferromagnetism, which is a prerequisite for use as retaining ring material in the construction of turbine generators. Owing to the high stability of its face-centered cubic crystal lattice, the material according to the invention does not exhibit any deformation martensite even after considerable cold working and therefore remains free of ferromagnetism.
The particular chemical properties of the material which has been subjected to considerable cold working according to the invention manifest themselves in the resistance to stress corrosion cracking in water and aqueous halide solutions. Other cold worked, nonferromagnetic, corrosion-resistant materials, even into the class of the xe2x80x9csuperaustenitesxe2x80x9d, but in particular all steels which have hitherto been conventional in engineering for retaining rings, have always proven susceptible to stress corrosion cracking in the high-strength, cold worked state, at least in hot, aqueous chloride solutions. This is the first time, with the high degree of cold deformation of 20% and more according to the invention, applied to said chromium alloy, that a material has been created which is entirely resistant to stress corrosion cracking in aqueous halide solutions even while retaining extremely high strength, resistance to corrosion and, at the same time, an absence of ferromagnetism.
By means of said method, the present invention has provided a material which, owing to its excellent combination of mechanical strength and toughness as well as corrosion resistance and its resistance to stress corrosion cracking, as well as the absence of ferromagnetism, can be used specifically in the following application areas: power engineering, offshore engineering and oil-drilling engineering, aeronautical and aerospatial engineering, the building and construction industry, general mechanical engineering, the chemical and petrochemical industries.